Decreased synthesis and vasodilation to nitric oxide in piglets with hypoxia-induced pulmonary hypertension

Is Inducible Nitric Oxide Synthase (iNOS) Promising as a New Target Against Pulmonary Hypertension?

Pulmonary hypertension (PH) is a progressive disease characterized by elevated blood pressure in the pulmonary arteries, associated also with inflammation and oxidative stress. Inducible nitric oxide synthase (iNOS) is one of the key mediators of inflammation and immune system activation. Although preclinical studies mostly suggest a detrimental role of iNOS overactivation in PH, there is a lack of exhaustive analyses and summaries. Therefore, this literature overview aims to fill this gap. The involvement of iNOS in the pathogenesis of the four main clinical groups of PH is discussed to assess whether targeting iNOS could be a promising way to treat PH. iNOS expression patterns in the organs primarily affected by PH are analyzed both in animals and in humans. Consequently, the effectiveness of pharmacological iNOS inhibition and/or iNOS gene deletion is discussed and compared, also with reference to the activity of constitutive NOS isoforms, particularly endothelial NOS (eNOS). Overall, our overview suggests that selective iNOS inhibitors could be considered as a novel treatment strategy for PH, as decreases in right ventricular and pulmonary artery pressure, the alleviation of ventricular hypertrophy, and improvements of pulmonary and cardiac function were observed, among others. Nevertheless, further research efforts in this area are needed.

Cardiovascular Sequelae of Bronchopulmonary Dysplasia in Preterm Neonates Born before 32 Weeks of Gestational Age: Impact of Associated Pulmonary and Systemic Hypertension

Bronchopulmonary dysplasia (BPD) remains the most common respiratory disorder of prematurity for infants born before 32 weeks of gestational age (GA). Early and prolonged exposure to chronic hypoxia and inflammation induces pulmonary hypertension (PH) with the characteristic features of a reduced number and increased muscularisation of the pulmonary arteries resulting in an increase in the pulmonary vascular resistance (PVR) and a fall in their compliance. BPD and BPD-associated pulmonary hypertension (BPD-PH) together with systemic hypertension (sHTN) are chronic cardiopulmonary disorders which result in an increased mortality and long-term problems for these infants. Previous studies have predominantly focused on the pulmonary circulation (right ventricle and its function) and developing management strategies accordingly for BPD-PH. However, recent work has drawn attention to the importance of the left-sided cardiac function and its impact on BPD in a subset of infants arising from a unique pathophysiology termed postcapillary PH. BPD infants may have a mechanistic link arising from chronic inflammation, cytokines, oxidative stress, catecholamines, and renin-angiotensin system activation along with systemic arterial stiffness, all of which contribute to the development of BPD-sHTN. The focus for the treatment of BPD-PH has been improvement of the right heart function through pulmonary vasodilators. BPD-sHTN and a subset of postcapillary PH may benefit from afterload reducing agents such as angiotensin converting enzyme inhibitors. Preterm infants with BPD-PH are at risk of later cardiac and respiratory morbidities as young adults. This paper reviews the current knowledge of the pathophysiology, diagnosis, and treatment of BPD-PH and BPD-sHTN. Current knowledge gaps and emerging new therapies will also be discussed.

Changes in the nitric oxide pathway of the pulmonary vasculature after exposure to hypoxia in swine model of neonatal pulmonary vascular disease

Neonatal pulmonary vascular disease (PVD) is increasingly recognized as a disease that complicates the cardiopulmonary adaptations after birth and predisposes to long-term cardiopulmonary disease. There is growing evidence that PVD is associated with disruptions in the nitric oxide (NO)-cGMP-phosphodiesterase 5 (PDE5) pathway. Examination of the functionality of different parts of this pathway is required for better understanding of the pathogenesis of neonatal PVD. For this purpose, the role of the NO-cGMP-PDE5 pathway in regulation of pulmonary vascular function was investigated in vivo, both at rest and during exercise, and in isolated pulmonary small arteries in vitro, in a neonatal swine model with hypoxia-induced PVD. Endothelium-dependent vasodilatation was impaired in piglets with hypoxia-induced PVD both in vivo at rest and in vitro. Moreover, the responsiveness to the NO-donor SNP was reduced in hypoxia-exposed piglets in vivo, while the relaxation to SNP and 8-bromo-cyclicGMP in vitro were unaltered. Finally, PDE5 inhibition-induced pulmonary vasodilatation was impaired in hypoxia-exposed piglets both in vitro and in vivo at rest. During exercise, however, the pulmonary vasodilator effect of PDE5 inhibition was significantly larger in hypoxia-exposed as compared to normoxia-exposed piglets. In conclusion, the impaired endothelium-dependent vasodilatation in piglets with hypoxia-induced PVD was accompanied by reduced responsiveness to NO, potentially caused by altered sensitivity and/or activity of soluble guanylyl cyclase (sGC), resulting in an impaired cGMP production. Our findings in a newborn animal model for neonatal PVD suggests that sGC stimulators/activators may be a novel treatment strategy to alleviate neonatal PVD.

Challenges in the development of chronic pulmonary hypertension models in large animals

Pulmonary hypertension (PH) results in significant morbidity and mortality. Chronic PH animal models may advance the study of PH's mechanisms, evolution, and therapy. In this report, we describe the challenges and successes in developing three models of chronic PH in large animals: two models (one canine and one swine) utilized repeated infusions of ceramic microspheres into the pulmonary vascular bed, and the third model employed a surgical aorto-pulmonary shunt. In the canine model, seven dogs underwent microsphere infusions that resulted in progressive elevation of pulmonary arterial pressure over a few months. In this model, pulmonary endoarterial tissue was obtained for histology. In the aorto-pulmonary shunt swine model, 17 pigs developed systemic level pulmonary pressures after 2-3 months. In this model, pulmonary endoarterial tissue was sequentially obtained to assess for changes in gene and microRNA expression. In the swine microsphere infusion model, three pigs developed only a modest chronic increase in pulmonary arterial pressure, despite repeated infusions of microspheres (up to 40 in one animal). The main purpose of this model was for vasodilator testing, which was performed successfully immediately after acute microsphere infusions. Chronic PH in large animal models can be successfully created; however, a model's characteristics need to match the investigational goals.

Megakaryocytic leukemia 1 (MKL1) regulates hypoxia induced pulmonary hypertension in rats

Hypoxia induced pulmonary hypertension (HPH) represents a complex pathology that involves active vascular remodeling, loss of vascular tone, enhanced pulmonary inflammation, and increased deposition of extracellular matrix proteins. Megakaryocytic leukemia 1 (MKL1) is a transcriptional regulator known to influence cellular response to stress signals in the vasculature. We report here that in response to chronic hypobaric hypoxia, MKL1 expression was up-regulated in the lungs in rats. Short hairpin RNA (shRNA) mediated depletion of MKL1 significantly ameliorated the elevation of pulmonary arterial pressure in vivo with a marked alleviation of vascular remodeling. MKL1 silencing also restored the expression of NO, a key vasoactive molecule necessary for the maintenance of vascular tone. In addition, hypoxia induced pulmonary inflammation was dampened in the absence of MKL1 as evidenced by normalized levels of pro-inflammatory cytokines and chemokines as well as reduced infiltration of pro-inflammatory immune cells in the lungs. Of note, MKL1 knockdown attenuated fibrogenesis in the lungs as indicated by picrosirius red staining. Finally, we demonstrate that MKL1 mediated transcriptional activation of type I collagen genes in smooth muscle cells under hypoxic conditions. In conclusion, we data highlight a previously unidentified role for MKL1 in the pathogenesis of HPH and as such lay down groundwork for future investigation and drug development.

Hypoxic pulmonary hypertension of the newborn

Hypoxic pulmonary hypertension of the newborn is characterized by elevated pulmonary vascular resistance and pressure due to vascular remodeling and increased vessel tension secondary to chronic hypoxia during the fetal and newborn period. In comparison to the adult, the pulmonary vasculature of the fetus and the newborn undergoes tremendous developmental changes that increase susceptibility to a hypoxic insult. Substantial evidence indicates that chronic hypoxia alters the production and responsiveness of various vasoactive agents such as endothelium-derived nitric oxide, endothelin-1, prostanoids, platelet-activating factor, and reactive oxygen species, resulting in sustained vasoconstriction and vascular remodeling. These changes occur in most cell types within the vascular wall, particularly endothelial and smooth muscle cells. At the cellular level, suppressed nitric oxide-cGMP signaling and augmented RhoA-Rho kinase signaling appear to be critical to the development of hypoxic pulmonary hypertension of the newborn.

Impaired iNOS-sGC-cGMP signalling contributes to chronic hypoxic and hypercapnic pulmonary hypertension in rat

Nitric oxide (NO) is an important vascular modulator in the development of pulmonary hypertension. NO exerts its regulatory effect mainly by activating soluble guanylate cyclase (sGC) to synthesize cyclic guanosine monophosphate (cGMP). Exposure to hypoxia causes pulmonary hypertension. But in lung disease, hypoxia is commonly accompanied by hypercapnia. The aim of this study was to examine the changes of sGC enzyme activity and cGMP content in lung tissue, as well as the expression of inducible nitric oxide synthase (iNOS) and sGC in rat pulmonary artery after exposure to hypoxia and hypercapnia, and assess the role of iNOS-sGC-cGMP signal pathway in the development of hypoxic and hypercapnic pulmonary hypertension. Male Sprague-Dawley rats were exposed to hypoxia and hypercapnia for 4 weeks to establish model of chronic pulmonary hypertension. Weight-matched rats exposed to normoxia served as control. After exposure to hypoxia and hypercapnia, mean pulmonary artery pressure, the ratio of right ventricle/left ventricle+septum, and the ratio of right ventricle/body weight were significantly increased. iNOS mRNA and protein levels were significantly increased, but sGC α(1) mRNA and protein levels were significantly decreased in small pulmonary arteries of hypoxic and hypercapnic exposed rat. In addition, basal and stimulated sGC enzyme activity and cGMP content in lung tissue were significantly lower after exposure to hypoxia and hypercapnia. These results demonstrate that hypoxia and hypercapnia lead to the upregulation of iNOS expression, downregulation of sGC expression and activity, which then contribute to the development of pulmonary hypertension.

Attenuated vasodilatation in lambs with endogenous and exogenous activation of cGMP signaling: role of protein kinase G nitration

Pulmonary vasodilation is mediated through the activation of protein kinase G (PKG) via a signaling pathway involving nitric oxide (NO), natriuretic peptides (NP), and cyclic guanosine monophosphate (cGMP). In pulmonary hypertension secondary to congenital heart disease, this pathway is endogenously activated by an early vascular upregulation of NO and increased myocardial B-type NP expression and release. In the treatment of pulmonary hypertension, this pathway is exogenously activated using inhaled NO or other pharmacological agents. Despite this activation of cGMP, vascular dysfunction is present, suggesting that NO-cGMP independent mechanisms are involved and were the focus of this study. Exposure of pulmonary artery endothelial or smooth muscle cells to the NO donor, Spermine NONOate (SpNONOate), increased peroxynitrite (ONOO(-) ) generation and PKG-1α nitration, while PKG-1α activity was decreased. These changes were prevented by superoxide dismutase (SOD) or manganese(III)tetrakis(1-methyl-4-pyridyl)porphyrin (MnTMPyP) and mimicked by the ONOO(-) donor, 3-morpholinosydnonimine N-ethylcarbamide (SIN-1). Peripheral lung extracts from 4-week old lambs with increased pulmonary blood flow and pulmonary hypertension (Shunt lambs with endogenous activation of cGMP) or juvenile lambs treated with inhaled NO for 24 h (with exogenous activation of cGMP) revealed increased ONOO(-) levels, elevated PKG-1α nitration, and decreased kinase activity without changes in PKG-1α protein levels. However, in Shunt lambs treated with L-arginine or lambs administered polyethylene glycol conjugated-SOD (PEG-SOD) during inhaled NO exposure, ONOO(-) and PKG-1α nitration were diminished and kinase activity was preserved. Together our data reveal that vascular dysfunction can occur, despite elevated levels of cGMP, due to PKG-1α nitration and subsequent attenuation of activity.

Pulmonary vascular wall stiffness: An important contributor to the increased right ventricular afterload with pulmonary hypertension

Pulmonary hypertension (PH) is associated with structural and mechanical changes in the pulmonary vascular bed that increase right ventricular (RV) afterload. These changes, characterized by narrowing and stiffening, occur in both proximal and distal pulmonary arteries (PAs). An important consequence of arterial narrowing is increased pulmonary vascular resistance (PVR). Arterial stiffening, which can occur in both the proximal and distal pulmonary arteries, is an important index of disease progression and is a significant contributor to increased RV afterload in PH. In particular, arterial narrowing and stiffening increase the RV afterload by increasing steady and oscillatory RV work, respectively. Here we review the current state of knowledge of the causes and consequences of pulmonary arterial stiffening in PH and its impact on RV function. We review direct and indirect techniques for measuring proximal and distal pulmonary arterial stiffness, measures of arterial stiffness including elastic modulus, incremental elastic modulus, stiffness coefficient β and others, the changes in cellular function and the extracellular matrix proteins that contribute to pulmonary arterial stiffening, the consequences of PA stiffening for RV function and the clinical implications of pulmonary vascular stiffening for PH progression. Future investigation of the relationship between PA stiffening and RV dysfunction may facilitate new therapies aimed at improving RV function and thus ultimately reducing mortality in PH.

Regulation of the Pulmonary Circulation in the Fetus and Newborn

During the development of the pulmonary vasculature in the fetus, many structural and functional changes occur to prepare the lung for the transition to air breathing. The development of the pulmonary circulation is genetically controlled by an array of mitogenic factors in a temporo-spatial order. With advancing gestation, pulmonary vessels acquire increased vasoreactivity. The fetal pulmonary vasculature is exposed to a low oxygen tension environment that promotes high intrinsic myogenic tone and high vasocontractility. At birth, a dramatic reduction in pulmonary arterial pressure and resistance occurs with an increase in oxygen tension and blood flow. The striking hemodynamic differences in the pulmonary circulation of the fetus and newborn are regulated by various factors and vasoactive agents. Among them, nitric oxide, endothelin-1, and prostaglandin I2 are mainly derived from endothelial cells and exert their effects via cGMP, cAMP, and Rho kinase signaling pathways. Alterations in these signaling pathways may lead to vascular remodeling, high vasocontractility, and persistent pulmonary hypertension of the newborn.

Changes in pulmonary arterial pressure in term-infants with hypoxic-ischemic encephalopathy

BACKGROUND

Hypoxic-ischemic encephalopathy (HIE) is an important complication that results from birth asphyxia or some other adverse conditions and has a high risk of neonatal morbidity and mortality. It is unclear, however, whether the elevated pulmonary arterial pressure (PAP) can aggravate the condition and prognosis of HIE. The purpose of the present study was to investigate the relationship between the changes of PAP and HIE in term infants after birth asphyxia.

METHODS

The left/right ventricle pre-ejection phase (LPEP/RPEP), left/right ventricle ejection time (LVET/RVET) and the ratios of LPEP/LVET and RPET/RVET were evaluated in 40 term infants with HIE and 40 healthy controls on days 1, 3, 7, and 12-14 after birth using echocardiogram. PAP such as pulmonary arterial diastolic pressure (PADP, mmHg), pulmonary arterial resistance (PAR, mmHg), and pulmonary arterial resistance/systemic resistance ratio (PAR/RS) was calculated using these indexes. Patient mortality was also evaluated.

RESULTS

PADP, PAR, and PAR/RS were significantly higher in HIE patients than in healthy controls during the first week after birth, particularly in severe-degree HIE patients. And until the end of the first week of life, these indexes may return to the levels of healthy controls. Persistent fetal circulation (PFC) was found in nine patients (7/16 severe, 2/12 moderate HIE patients), and non-PFC was found in mild HIE patients. Two patients with PFC died. No patients without PFC died. The course of HIE was longer in patients with pulmonary hypertension than in those without.

CONCLUSION

Increased PAP is an important pathophysiological process that may influence the course and prognoses of HIE in infants after birth asphyxia, particular in severe HIE patients who often have PFC. Thus it is important to assess changes in PAP using echocardiography.

Effects of chronic hypoxia on soluble guanylate cyclase activity in fetal and adult ovine cerebral arteries

A broad variety of evidence obtained largely in pulmonary vasculature suggests that chronic hypoxia modulates vasoreactivity to nitric oxide (NO). The present study explores the general hypothesis that chronic hypoxia also modulates cerebrovascular reactivity to NO, and does so by modulating the activity of soluble guanylate cyclase (sGC), the primary target for NO in vascular smooth muscle. Pregnant and nonpregnant ewes were maintained at either sea level or at 3,820 m for the final 110 days of gestation, at which time middle cerebral arteries from term fetal lambs and nonpregnant adults were harvested. In both fetal and adult arteries, NO-induced vasodilatation was attenuated by chronic hypoxia and completely inhibited by 10 microM 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ), a selective inhibitor of sGC. sGC abundance (in ng sGC/mg protein) measured via Western immunoblots was approximately 10-fold greater in fetal (17.6 +/- 1.6) than adult (1.7 +/- 0.3) arteries but was not affected by chronic hypoxia. The specific activity of sGC (in pmol cGMP.microg sGC(-1).min(-1)) was similar in fetal (255 +/- 64) and adult (280 +/- 75) arteries and was inhibited by chronic hypoxia in both fetal (120 +/- 10) and adult (132 +/- 26) arteries. Rates of cGMP degradation (in pmol cGMP.mg protein(-1).min(-1)) were similar in fetal (159 +/- 59) and adult (134 +/- 36) arteries but were not significantly depressed by chronic hypoxia in either fetal (115 +/- 25) or adult (108 +/- 25) arteries. The cGMP analog 8-(p-chlorophenylthio)-cGMP was a more potent vasorelaxant in fetal (pD(2) = 4.7 +/- 0.1) than adult (pD(2) = 4.3 +/- 0.1) arteries, but its ability to promote vasodilatation was not affected by chronic hypoxia in either age group. Together, these results reveal that hypoxic inhibition of NO-induced vasodilatation is attributable largely to attenuation of the specific activity of sGC and does not involve significant changes in sGC abundance, cGMP-phosphodiesterase activity, or the vasorelaxant activity of protein kinase G.

Heat shock protein 90-eNOS interactions mature with postnatal age in the pulmonary circulation of the piglet

Binding of endothelial nitric oxide synthase (eNOS) to the chaperone protein, Hsp90, promotes coupled eNOS synthetic activity. Using resistance level pulmonary arteries (PRA) from 2-day-, 5- to 7-day-, and 12-day-old piglets, we tested the hypothesis that Hsp90-eNOS interactions are developmentally regulated in the early neonatal period. PRA were isolated for coimmunoprecipitation and immunoblot analyses or cannulated for continuous diameter measurements using the pressurized myography technique. NOS inhibition caused less constriction in PRA from 2-day- compared with 5- to 7-day- and 12-day-old piglets. No age-related differences were found in dilation responses to an NO donor or in protein expression of Hsp90, phospho-eNOS (Ser(1177)), Akt, phospho-Akt, or caveolin-1. Compared with the older animals, PRA from 2-day-old piglets had higher total eNOS expression but displayed less binding of eNOS to Hsp90 and Akt. Hsp90 antagonism with radicicol induced greatest constriction in PRA from 12-day-old piglets. ACh stimulation caused dilation in PRA from 5- to 7-day- and 12-day-old but not 2-day-old animals, despite rapid and equivalent ACh-mediated eNOS phosphorylation (Ser(1177)) in all three age groups. Hsp90 inhibition abolished ACh-mediated dilation in PRA from the older piglets. ACh failed to stimulate Hsp90-eNOS binding in 2-day-old but induced a significant increase in Hsp90-eNOS coimmunoprecipitation in PRA from the older age groups, which was blocked by Hsp90 antagonism. We conclude that physical interactions between Hsp90 and eNOS mature over the first weeks of life, likely contributing to the postnatal fall in pulmonary vascular resistance and changes in agonist-induced pulmonary vascular responses characteristic of the early neonatal period.

Upregulation of vascular calcium channels in neonatal piglets with hypoxia-induced pulmonary hypertension

Inhibition of voltage-gated, L-type Ca(2+) (Ca(L)) channels by clinical calcium channel blockers provides symptomatic improvement to some pediatric patients with pulmonary arterial hypertension (PAH). The present study investigated whether abnormalities of vascular Ca(L) channels contribute to the pathogenesis of neonatal PAH using a newborn piglet model of hypoxia-induced PAH. Neonatal piglets exposed to chronic hypoxia (CH) developed PAH by 21 days, which was evident as a 2.1-fold increase in pulmonary vascular resistance in vivo compared with piglets raised in normoxia (N). Transpulmonary pressures (DeltaPtp) in the corresponding isolated perfused lungs were 20.5 +/- 2.1 mmHg (CH) and 11.6 +/- 0.8 mmHg (N). Nifedipine reduced the elevated DeltaPtp in isolated lungs of CH piglets by 6.4 +/- 1.3 mmHg but only reduced DeltaPtp in lungs of N piglets by 1.9 +/- 0.2 mmHg. Small pulmonary arteries from CH piglets also demonstrated accentuated Ca(2+)-dependent contraction, and Ca(2+) channel current was 3.94-fold higher in the resident vascular muscle cells. Finally, although the level of mRNA encoding the pore-forming alpha(1C)-subunit of the Ca(L) channel was similar between small pulmonary arteries from N and CH piglets, a profound and persistent upregulation of the vascular alpha(1C) protein was detected by 10 days in CH piglets at a time when pulmonary vascular resistance was only mildly elevated. Thus chronic hypoxia in the neonate is associated with the anomalous upregulation of Ca(L) channels in small pulmonary arteries in vivo and the resulting abnormal Ca(2+)-dependent resistance may contribute to the pathogenesis of PAH.

Pathobiology of pulmonary hypertension

A variety of conditions can lead to the development of pulmonary arterial hypertension (PAH). Current treatments can improve symptoms and reduce the severity of the hemodynamic abnormality, but most patients remain quite limited, and deterioration in their condition necessitates a lung transplant. This review discusses current experimental and clinical studies that investigate the pathobiology of PAH. An emerging theme is the consideration of ways in which one might reverse the advanced occlusive structural changes in the pulmonary circulation causing PAH. The current debate concerning the role of regeneration through stem cells is presented. This review also highlights investigations in a number of laboratories relating the pathobiology of PAH to mutations causing loss of function of bone morphogenetic protein receptor II in patients with familial PAH, as well as sporadic cases.

Postnatal changes in pulmonary vein responses to endothelin-1 in the normal and chronically hypoxic lung

The response of pulmonary arteries to endothelin-1 (ET-1) changes with age in normal pigs and is abnormal in pulmonary hypertension. The purpose of this study was to determine if the same is true of the pulmonary veins. We studied the wall structure and functional response to ET-1 in pulmonary veins from normal pigs from fetal life to adulthood and from pigs subjected to chronic hypobaric hypoxia either from birth for 3 days or from 3 to 6 days of age. In isolated normal veins, the contractile response decreased by 40% between late fetal life and 14 days of age with a concomitant twofold increase in endothelium-dependent relaxant response. The ETA antagonist BQ-123 reduced the contractile response significantly more in newborn than older animals, whereas the ET-B antagonist BQ-788 had no effect in fetal animals and maximally increased contraction at 14 days of age. Hypoxic exposure significantly increased pulmonary vein smooth muscle area and contractile response to ET-1. The relaxation response was impaired following hypoxic exposure from birth but not from 3 to 6 days of age. The ETA antagonist BQ-123 decreased contractile and increased dilator responses significantly more than in age-matched controls. Thus pulmonary veins show age-related changes similar to those seen in the pulmonary arteries with a decrease in ETA-mediated contractile and increase in ET-B-mediated relaxant response with age. Contractile response was also increased in hypoxia as in the arteries. This study suggests that pulmonary veins are involved in postnatal adaptation and the pathogenesis of pulmonary hypertension.

Role of Rho kinases in PKG-mediated relaxation of pulmonary arteries of fetal lambs exposed to chronic high altitude hypoxia

An increase in Rho kinase (ROCK) activity is implicated in chronic hypoxia-induced pulmonary hypertension. In the present study, we determined the role of ROCKs in cGMP-dependent protein kinase (PKG)-mediated pulmonary vasodilation of fetal lambs exposed to chronic hypoxia. Fourth generation pulmonary arteries were isolated from near-term fetuses ( approximately 140 days of gestation) delivered from ewes exposed to chronic high altitude hypoxia for approximately 110 days and from control ewes. In vessels constricted to endothelin-1, 8-bromoguanosine-cGMP (8-Br-cGMP) caused a smaller relaxation in chronically hypoxic (CH) vessels compared with controls. Rp-8-Br-PET-cGMPS, a PKG inhibitor, attenuated relaxation to 8-Br-cGMP in control vessels to a greater extent than in CH vessels. Y-27632, a ROCK inhibitor, significantly potentiated 8-Br-cGMP-induced relaxation of CH vessels and had only a minor effect in control vessels. The expression of PKG was increased but was not accompanied with an increase in the activity of the enzyme in CH vessels. The expression of type II ROCK and activity of ROCKs were increased in CH vessels. The phosphorylation of threonine (Thr)696 and Thr850 of the regulatory subunit MYPT1 of myosin light chain phosphatase was inhibited by 8-Br-cGMP to a lesser extent in CH vessels than in controls. The difference was eliminated by Y-27632. These results suggest that chronic hypoxia in utero attenuates PKG-mediated relaxation in pulmonary arteries, partly due to inhibition of PKG activity and partly due to enhanced ROCK activity. Increased ROCK activity may inhibit PKG action through increased phosphorylation of MYPT1 at Thr696 and Thr850.

Persistent pulmonary hypertension of the newborn

In utero, fetal pulmonary vascular resistance (PVR) is high, but rapidly falls after birth. Expansion of the lungs, increase in oxygenation, release of vasoactive mediators, growth factors and remodeling of the vascular wall, all contribute to the reduction in PVR. Persistent pulmonary hypertension of the newborn (PPHN) is defined as a failure of the pulmonary vasculature to relax at birth, resulting in hypoxemia. PPHN is in fact a variety of disorders that have a common presentation. Some of the pathophysiological mechanisms and the therapeutic approaches are discussed below.

Deletion of the eNOS gene has a greater impact on the pulmonary circulation of male than female mice

Nitric oxide is involved in development and postnatal adaptation of the pulmonary circulation. This study aimed to determine whether genetic deletion of nitric oxide synthase (NOS) would lead to maldevelopment of the pulmonary arteries in fetal life, compromise adaptation to extrauterine life, and be associated with a pulmonary hypertensive phenotype in adult life and if any abnormalities were detected, were they sex dependent. Morphometric analyses were made on lung tissue from male and female fetal, newborn, 14-day-old, and adult endothelial NOS-deficient (eNOS−/−) or inducible NOS-deficient (iNOS−/−) and wild-type mice. Hemodynamic studies were carried out on adult mice with deletion of either eNOS or iNOS genes. We found that in eNOS−/− mice, lung development was normal in fetal, newborn, and adult lungs. Pulmonary arterial muscularity was greater than normal in both male and female eNOS−/− during fetal life and at birth, but the abnormality persisted only in male mice. Right ventricular hypertrophy was present in 14-day-old and adult male eNOS−/− but not in female mice. Adult male eNOS−/− mice had higher mean right ventricular and systemic pressures than female eNOS−/− mice ( P < 0.05). Thus deletion of the eNOS gene was associated with structural evidence of pulmonary hypertension in both sexes during fetal life, but pulmonary hypertension persisted only in the male. In neither sex did iNOS or neuronal NOS appear to compensate for the eNOS deletion. Adult iNOS−/− mice did not have structural or hemodynamic evidence of pulmonary hypertension. Possible compensatory mechanisms are discussed.

Pathophysiologic mechanisms of persistent pulmonary hypertension of the newborn

Persistent pulmonary hypertension of the newborn (PPHN), among the most rapidly progressive and potentially fatal of vasculopathies, is a disorder of vascular transition from fetal to neonatal circulation, manifesting as hypoxemic respiratory failure. PPHN represents a common pathway of vascular injury activated by numerous perinatal stresses: hypoxia, hypoglycemia, cold stress, sepsis, and direct lung injury. As with other multifactorial diseases, a single inciting event may be augmented by multiple concurrent/subsequent phenomena that result in differing courses of disease progression. I review the various mechanisms of vascular injury involved in neonatal pulmonary hypertension: endothelial dysfunction, inflammation, hypoxia, and mechanical strain, in the context of downstream effects on pulmonary vascular endothelial-myocyte interactions and myocyte phenotypic plasticity.

Thromboxane inhibition reduces an early stage of chronic hypoxia-induced pulmonary hypertension in piglets

The pulmonary vasoconstrictor, thromboxane, may contribute to the development of pulmonary hypertension. Our objective was to determine whether a combined thromboxane synthase inhibitor-receptor antagonist, terbogrel, prevents pulmonary hypertension and the development of aberrant pulmonary arterial responses in newborn piglets exposed to 3 days of hypoxia. Piglets were maintained in room air (control) or 11% O(2) (hypoxic) for 3 days. Some hypoxic piglets received terbogrel (10 mg/kg po bid). Pulmonary arterial pressure, pulmonary wedge pressure, and cardiac output were measured in anesthetized animals. A cannulated artery technique was used to measure responses to acetylcholine. Pulmonary vascular resistance for terbogrel-treated hypoxic piglets was almost one-half the value of untreated hypoxic piglets but remained greater than values for control piglets. Dilation to acetylcholine in preconstricted pulmonary arteries was greater for terbogrel-treated hypoxic than for untreated hypoxic piglets, but it was less for pulmonary arteries from both groups of hypoxic piglets than for control piglets. Terbogrel may ameliorate pulmonary artery dysfunction and attenuate the development of chronic hypoxia-induced pulmonary hypertension in newborns.

Raloxifene relaxes rat pulmonary arteries and veins: roles of gender, endothelium, and antagonism of Ca2+ influx

Effects of raloxifene have been documented in the systemic circulation. However, its impact on the pulmonary circulation is unclear. The present study investigated the role of gender, endothelial modulation, and Ca(2+) channel in relaxations evoked by raloxifene in rat pulmonary arteries and veins. Vascular responses were studied on isolated pulmonary blood vessels mounted in a myograph and constricted by U46619 (9,11-dideoxy-11alpha,9alpha-epoxymethanoprostaglandin F(2alpha)). Constrictions to CaCl(2) were studied in Ca(2+)-free, 60 mM K(+) solution. Changes in the intracellular calcium ion concentration ([Ca(2+)](i)) in vascular smooth muscle were measured using a calcium fluorescence imaging method. Raloxifene was more effective in relaxing U46619-constricted pulmonary arteries from male than female rats. Raloxifene-induced relaxation was unaffected by ICI 182,780 [7alpha-[9-[(4,4,5,5,5,-pentafluoropentyl)-sulfinyl]nonyl]-estra-1,3,5(10)-triene-3,17beta-diol], inhibition of the nitric oxide (NO) pathway, or removal of the endothelium. In arteries without endothelium, raloxifene attenuated CaCl(2)-induced constriction and CaCl(2)-stimulated increase in [Ca(2+)](i) with similar potencies. Raloxifene caused endothelium-independent relaxations in pulmonary veins, albeit to a lesser degree than in pulmonary arteries. The venous responses showed a gender difference because raloxifene was more potent in male veins. In summary, raloxifene relaxed rat pulmonary arteries, and this effect did not involve the endothelium/NO or ICI 182,780-sensitive estrogen receptors. Raloxifene, like nifedipine, reduced constriction and [Ca(2+)](i) increase in response to CaCl(2) in high K(+) solution. Raloxifene also relaxed high K(+)-constricted pulmonary veins. Our data indicate that raloxifene acutely relaxes rat pulmonary blood vessels primarily via inhibition of Ca(2+) influx through voltage-sensitive Ca(2+) channels. Finally, raloxifene induced more relaxation in blood vessels isolated from male than female rats.

Postnatal Changes in Response to Norepinephrine in the Normal and Pulmonary Hypertensive Lung

The effect of norepinephrine administration on pulmonary blood flow during the neonatal period is unclear. Therefore, norepinephrine responses were studied in isolated pulmonary arteries, pulmonary veins, and femoral arteries taken from normal pigs from birth to adulthood and from pigs subjected to chronic hypoxia either from birth for 3 days or from 3 to 14 days of age. Normally, the contractile response of pulmonary arteries and veins to norepinephrine decreased after birth (p < 0.01), and alpha2-adrenoceptor-mediated relaxation increased in pulmonary arteries and veins and in femoral arteries. Hypoxic exposure from birth prevented the normal postnatal reduction in pulmonary arterial contractile response, nor was there a postnatal increase in pulmonary arterial adrenoceptor-mediated relaxation. When hypoxic exposure followed a period of normal adaptation, the pulmonary arterial contractile response was not enhanced, but relaxation was significantly impaired. The response of pulmonary veins and femoral arteries was not affected by hypoxic exposure. The contractile effect of norepinephrine was 15- to 60-fold greater in isolated systemic arteries than in pulmonary arteries taken from both normal and pulmonary hypertensive piglets at all ages. This suggests that use of norepinephrine to manage systemic hypotension in infants and children will not compromise the pulmonary vasculature.

Molecular biology of primary pulmonary hypertension

Primary pulmonary hypertension (PPH) is a rare but often fatal condition characterized by pulmonary artery remodeling leading to chronic elevation of pulmonary artery pressure in the absence of causes. The pathophysiology of PPH is not completely understood, but a number of recent studies have elucidated many possible gentic, hormonal, and environmental factors. Current treatment options slow the progression of the disease but do not halt it. The study of molecular mechanisms that result from mutations in onmental and hormonal modifiers holds great promise for the development of novel therapies that may halt the progression of the disease.

Impaired NO signaling in small pulmonary arteries of chronically hypoxic newborn piglets

We performed studies to determine whether chronic hypoxia impairs nitric oxide (NO) signaling in resistance level pulmonary arteries (PAs) of newborn piglets. Piglets were maintained in room air (control) or hypoxia (11% O2) for either 3 (shorter exposure) or 10 (longer exposure) days. Responses of PAs to a nonselective NO synthase (NOS) antagonist, Nω-nitro-l-arginine methylester (l-NAME), a NOS-2-selective antagonist, aminoguanidine, and 7-nitroindazole, a NOS-1-selective antagonist, were measured. Levels of NOS isoforms and of two proteins involved in NOS signaling, heat shock protein (HSP) 90 and caveolin-1, were assessed in PA homogenates. PAs from all groups constricted to l-NAME but not to aminoguanidine or 7-nitroindazole. The magnitude of constriction to l-NAME was similar for PAs from control and hypoxic piglets of the shorter exposure period but was diminished for PAs from hypoxic compared with control piglets of the longer exposure period. NOS-3, HSP90, and caveolin-1 levels were similar in hypoxic and control PAs. These findings indicate that NOS-3, but not-NOS 2 or NOS-1, is involved with basal NO production in PAs from both control and hypoxic piglets. After 10 days of hypoxia, NO function is impaired in PAs despite preserved levels of NOS-3, HSP90, and caveolin-1. The development of NOS-3 dysfunction in resistance level PAs may contribute to the progression of chronic hypoxia-induced pulmonary hypertension in newborn piglets.

Roles of Accumulated Endogenous Nitric Oxide Synthase Inhibitors and Decreased Nitric Oxide Synthase Activity for Impaired Trigonal Relaxation With Ischemia

PURPOSE

We examined whether endogenous nitric oxide (NO) synthase (NOS) inhibitors are involved in the impaired trigonal relaxation with ischemia in rabbits.

MATERIALS AND METHODS

Rabbits were divided into control and ischemia groups. Two weeks after partial vessel occlusion strips of trigone and detrusor were processed to determine endogenous methylarginines and L-arginine by automated high performance liquid chromatography. We also compared NOS activity and NO mediated functional responses to electrical field stimulation between 2 groups.

RESULTS

Neurogenic and NO but not sodium nitroprusside induced mediated relaxation in the trigone were significantly impaired following ischemia. Ca2+ dependent NOS activity, and baseline and stimulated cyclic guanosine monophosphate production with electrical field stimulation were significantly decreased following ischemia. The contents of L-NMMA (NG-monomethyl-L-arginine) and asymmetrical ADMA (NG, NG-dimethyl-L-arginine) but not L-arginine or symmetrical SDMA (NG, N'G-dimethyl-L-arginine) were increased in the trigone following ischemia. Authentic L-NMMA and ADMA but not SDMA inhibited neurogenic relaxations in a concentration dependent manner without affecting the relaxation produced by sodium nitroprusside in control tissue. Excess L-arginine abolished L-NMMA and ADMA inhibition.

CONCLUSIONS

These results suggest that impaired NO mediated trigonal relaxation following ischemia is closely related to decreased NOS activity and the increased accumulation of L-NMMA and ADMA.

Chronic O2 exposure enhances vascular and airway smooth muscle contraction in the newborn but not adult rat

Neonatal rats exposed to 60% O(2) for 14 days develop lung changes compatible with human bronchopulmonary dysplasia and pulmonary hypertension. Our aim was to evaluate and compare the newborn and adult rat pulmonary vascular and airway smooth muscle force generation and relaxation potential after exposure to 60% O(2) for 14 days. Vascular and airway intrapulmonary rings 100 microm in diameter were mounted on a myograph and bathed in Krebs-Henseleit solution bubbled with air- 6% CO(2) at 37 degrees C. Significant age-dependent changes in intrapulmonary arteries and their neighboring airway muscle properties were observed. Whereas hyperoxia enhanced force in neonatal vascular and airway muscle, the opposite was seen in adult samples. No changes in endothelium-dependent vascular relaxation were observed at either age, but the dose response to an endothelium-independent NO donor was altered. In the newborn experimental animals, the relaxation was reduced, whereas, in their adult counterparts, it was enhanced. After O(2) exposure, the bronchial muscle relaxation response to epithelium-dependent and -independent stimulation was not altered in either age group, whereas the epithelium-dependent response was decreased only in the adult. The antioxidant Trolox, or an endothelin-A and -B receptor antagonist, reversed the vascular and airway muscle's hyperoxia-induced changes. We conclude that chronic O(2) exposure in the newborn rat results in enhanced lung vascular and airway muscle contraction potential via a mechanism involving reactive oxygen species and the endothelin pathway. The present findings also suggest that the newborn is more susceptible to airway hyperresponsiveness after chronic O(2) exposure.

Exhaled NO is reduced at an early stage of hypoxia-induced pulmonary hypertension in newborn piglets

Altered nitric oxide (NO) production could contribute to the pathogenesis of hypoxia-induced pulmonary hypertension. To determine whether parameters of lung NO are altered at an early stage of hypoxia-induced pulmonary hypertension, newborn piglets were exposed to room air (control, n = 21) or 10% O(2) (hypoxia, n = 19) for 3-4 days. Some lungs were isolated and perfused for measurement of exhaled NO output and the perfusate accumulation of nitrite and nitrate (NOx-), the stable metabolites of NO. Pulmonary arteries (20-600-microm diameter) and their accompanying airways were dissected from other lungs and incubated for NOx- determination. Abundances of the nitric oxide synthase (NOS) isoforms endothelial NOS and neural NOS were assessed in homogenates of PAs and airways. The perfusate NOx- accumulation was similar, whereas exhaled NO output was lower for isolated lungs of hypoxic, compared with control, piglets. The incubation solution NOx- did not differ between pulmonary arteries (PAs) of the two groups but was lower for airways of hypoxic, compared with control, piglets. Abundances of both eNOS and nNOS proteins were similar for PA homogenates from the two groups of piglets but were increased in airway homogenates of hypoxic compared with controls. The NO pathway is altered in airways, but not in PAs, at an early stage of hypoxia-induced pulmonary hypertension in newborn piglets.

Oxygen-dependent signaling in pulmonary vascular smooth muscle

The pulmonary circulation constricts in response to acute hypoxia, which is reversible on reexposure to oxygen. On exposure to chronic hypoxia, in addition to vasoconstriction, the pulmonary vasculature undergoes remodeling, resulting in a sustained increase in pulmonary vascular resistance that is not immediately reversible. Hypoxic pulmonary vasoconstriction is physiological in the fetus, and there are many mechanisms by which the pulmonary vasculature relaxes at birth, principal among which is the acute increase in oxygen. Oxygen-induced signaling mechanisms, which result in pulmonary vascular relaxation at birth, and the mechanisms by which chronic hypoxia results in pulmonary vascular remodeling in the fetus and adult, are being investigated. Here, the roles of cGMP-dependent protein kinase in oxygen-mediated signaling in fetal pulmonary vascular smooth muscle and the effects of chronic hypoxia on ion channel activity and smooth muscle function such as contraction, growth, and gene expression were discussed.

Accumulated endogenous NOS inhibitors, decreased NOS activity, and impaired cavernosal relaxation with ischemia

We examined whether endogenous inhibitors of nitric oxide (NO) synthesis are involved in the impaired cavernosal relaxation with ischemia in rabbits. Two weeks after cavernosal ischemia caused by partial vessel occlusion, endothelium-dependent and electrical field stimulation (EFS)-induced neurogenic NO-mediated relaxations, but not sodium nitroprusside (SNP)-induced relaxation, were significantly impaired in the isolated corpus cavernosum. The Ca(2+)-dependent NO synthase (NOS) activity and the basal and stimulated cGMP productions with carbachol or EFS were significantly decreased after ischemia. Supplementation of excess L-arginine partially recovered both of the impaired relaxations. The contents of N(G)-monomethyl-L-arginine (L-NMMA) and asymmetric N(G), N(G)-dimethyl-L-arginine (ADMA) but not L-arginine and symmetric N(G),N'(G)-dimethyl-L-arginine (SDMA) were increased in the cavernosal tissues after ischemia. Authentic L-NMMA and ADMA but not SDMA concentration dependently inhibited both relaxations without affecting the relaxation produced by SNP in the control. Excess L-arginine abolished the inhibition with L-NMMA and ADMA. These results suggest that the impaired NO-mediated cavernosal relaxations after ischemia are closely related to the decreased NOS activity and the increased accumulation of L-NMMA and ADMA.

Chronic hypoxia attenuates cGMP-dependent pulmonary vasodilation

Chronic hypoxia (CH) augments endothelium-derived nitric oxide (NO)-dependent pulmonary vasodilation; however, responses to exogenous NO are reduced following CH in female rats. We hypothesized that CH-induced attenuation of NO-dependent pulmonary vasodilation is mediated by downregulation of vascular smooth muscle (VSM) soluble guanylyl cyclase (sGC) expression and/or activity, increased cGMP degradation by phosphodiesterase type 5 (PDE5), or decreased VSM sensitivity to cGMP. Experiments demonstrated attenuated vasodilatory responsiveness to the NO donors S-nitroso-N-acetylpenicillamine and spermine NONOate and to arterial boluses of dissolved NO solutions in isolated, saline-perfused lungs from CH vs. normoxic female rats. In additional experiments, the sGC inhibitor, 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one, blocked vasodilation to NO donors in lungs from each group. However, CH was not associated with decreased pulmonary sGC expression or activity as assessed by Western blotting and cGMP radioimmunoassay, respectively. Consistent with our hypothesis, the selective PDE5 inhibitors dipyridamole and T-1032 augmented NO-dependent reactivity in lungs from CH rats, while having little effect in lungs from normoxic rats. However, the attenuated vasodilatory response to NO in CH lungs persisted after PDE5 inhibition. Furthermore, CH similarly inhibited vasodilatory responses to 8-bromoguanosine 3'5'-cyclic monophosphate. We conclude that attenuated NO-dependent pulmonary vasodilation after CH is not likely mediated by decreased sGC expression, but rather by increased cGMP degradation by PDE5 and decreased pulmonary VSM reactivity to cGMP.

Cyclooxygenase contracting factors and altered pulmonary vascular responses in chronically hypoxic newborn pigs

Pulmonary hypertension and blunted pulmonary vascular responses to ACh develop when newborn pigs are exposed to chronic hypoxia for 3 days. To determine whether a cyclooxygenase (COX)-dependent contracting factor, such as thromboxane, is involved with altered pulmonary vascular responses to ACh, newborn piglets were raised in 11% O(2) (hypoxic) or room air (control) for 3 days. Small pulmonary arteries (100-400 microm diameter) were cannulated and pressurized, and their responses to ACh were measured before and after either the COX inhibitor indomethacin; a thromboxane synthesis inhibitor, dazoxiben or feregrelate; or the thromboxane-PGH(2)-receptor antagonist SQ-29548. In control arteries, indomethacin reversed ACh responses from dilation to constriction. In contrast, hypoxic arteries constricted to ACh before indomethacin and dilated to ACh after indomethacin. Furthermore, ACh constriction in hypoxic arteries was nearly abolished by either dazoxiben, feregrelate, or SQ-29548. These findings suggest that thromboxane is the COX-dependent contracting factor that underlies the constrictor response to ACh that develops in small pulmonary arteries of piglets exposed to 3 days of hypoxia. The early development of thromboxane-mediated constriction may contribute to the pathogenesis of chronic hypoxia-induced pulmonary hypertension in newborns.

Role of protein kinase C and phosphatases in the pulmonary vasculature of neonatal piglets

OBJECTIVE

Persistent pulmonary hypertension of the newborn is characterized by the presence of intense vasoconstriction and vascular remodeling. Protein tyrosine phosphorylation has been recognized as a critical regulatory element in signal transduction, because it is dynamically regulated by the opposing actions of protein tyrosine kinases and protein tyrosine phosphatases. The objectives of this study were to investigate the role of protein kinase C and phosphatases in the neonatal pulmonary vasculature of normoxic and chronically hypoxic piglets.

DESIGN

Prospective, randomized, unblinded study.

SETTING

Hospital research laboratory.

SUBJECTS

Newborn Yorkshire-Landrace piglets.

INTERVENTIONS

Normoxic animals were 3-6 days old. Hypoxic animals were exposed to hypoxia (Fio2 0.10) between 1 and 15 days of age to induce pulmonary hypertension and then were studied.

MEASUREMENTS AND MAIN RESULTS

In isolated perfused lungs from normoxic piglets, we measured the perfusion pressure to assess the vasoconstrictor response to protein kinase C activation with phorbol 12,13-dibutyrate or 1-oleyl-2-acetyl-glycerol. We also assessed the effect of protein kinase C inhibition with staurosporine (2 x 10-6M) and chelerythrine (5 x 10-5M) on endothelin-1-induced pulmonary vasoconstriction. We then examined the effect of chelerythrine and phosphatase inhibition with phenylarsine oxide on the baseline perfusion pressure of normoxic and chronically hypoxic piglets. Phorbol 12,13-dibutyrate and 1-oleyl-2-acetyl-glycerol caused a sustained, dose-dependent increase in perfusion pressure, with relative potencies about 100- and 1000-fold less than endothelin-1, respectively. Protein kinase C inhibitors, chelerythrine and staurosporine, decreased the constrictor response to endothelin-1. Chelerythrine did not affect baseline perfusion pressure in the normoxic animal, whereas it lowered pulmonary vascular tone in chronically hypoxic animals. With respect to phosphatases, phenylarsine oxide significantly increased perfusion pressure in normoxia as well as in hypoxia.

CONCLUSIONS

These findings confirm that protein kinase C activation causes sustained vasoconstriction in the neonatal pulmonary vasculature and mediates the vasoconstrictor action of potent peptides, like endothelin-1. These findings also confirm that protein kinase C activation could be induced by hypoxic exposure in the neonatal piglet pulmonary vasculature. Phosphatases appear to modulate pulmonary vascular tone in the normoxic and hypoxic newborn piglet.

Involvement of accumulated endogenous NOS inhibitors and decreased NOS activity in the impaired neurogenic relaxation of the rabbit proximal urethra with ischaemia

1. We examined the effect of ischaemia on the neurogenic and nitric oxide (NO)-mediated urethral relaxation. 2. Rabbits were divided into control and urethral ischaemia (UI) groups, which was prepared by the partial occlusion of bilateral iliac arteries using blood vessel occluders. 3. Neurogenic and NO-mediated proximal urethral relaxation induced by electrical field stimulation (EFS) was greatly impaired in the UI group, while relaxation by sodium nitroprusside (SNP) as a NO donor showed no difference between the two groups. Pretreatment with L-arginine significantly improved but did not normalize the impaired relaxation in the UI group. Not only basal level, but also stimulated production of cyclic GMP with EFS, were significantly decreased in the UI group. 4. The tissue contents of N(G)-methyl-L-arginine (L-NMA) and asymmetric N(G), N(G)-dimethyl-L-arginine (ADMA) in the proximal urethra were increased following ischaemia. While L-arginine and symmetric N(G), N'(G)-dimethyl-L-arginine (SDMA) contents remained unchanged. Exogenously applied authentic L-NMA and ADMA (1 -- 100 microM) concentration-dependently inhibited the EFS-induced urethral relaxation in the control group. The inhibition with L-NMA and ADMA was undetectable in the presence of 3 mM L-arginine. 5. The Ca(2+)-dependent NOS activity in the urethra from the UI group was significantly lower than that from the control group and was not restored by an addition of 3 mM L-arginine. 6. These results suggest that the impaired neurogenic and NO-mediated urethral relaxation with ischaemia is closely related to the increased accumulation of L-NMA and ADMA and decreased NOS activity, which would result in an accelerated reduction in NO production/release.